Hallmarks of aging that negatively impact health include weight gain and reduced physical fitness, which can increase insulin resistance and risk for many diseases including type 2 diabetes. The underlying mechanism(s) for these phenomena is poorly understood. Aging increases oxidative stress and DNA breaks and activates DNA-dependent protein kinase (DNA-PK) in skeletal muscle, which suppresses mitochondrial function, energy metabolism and physical fitness. We find that endonuclease G (EndoG), a mitochondrial nuclease, regulates DNA-PK in response to oxidative stress. Deficiency of EndoG increases DNA-PK activity at low levels of oxidative stress, but decreases DNA-PK at high levels of oxidative stress. This observation links DNA-PK activity to mitochondrial function, and we are in the process of elucidating the molecular mechanism for this link.